The present invention relates to an improved method of casting an article.
Molds for reactive metals have previously been formed of graphite to minimize reaction with the metal to be cast and to allow the temperature at which the metal is cast to be increased. In making the mold, a wax pattern or cluster is repetitively dipped in colloidal graphite and stuccoed with particles of graphite. Since the graphite in the resulting mold will be consumed when heated to a temperature above 430.degree. C., in an oxidizing atmosphere, dewaxing and firing are carried out in an inert or non-oxidizing atmosphere. The resulting mold is used for the casting of reactive or refractory metals such as zirconium, titanium, or uranium. The method of making these graphite molds is disclosed in U.S. Pat. No. 3,256,574. The concept of coating the inside or article forming surface of a mold with graphite to improve the characteristics of a cast article is disclosed in U.S. Pat. Nos. 2,618,032; 3,401,735; and 3,645,767.
During the casting of turbine blades and vanes from nickel chrome superalloys with ceramic molds made by the lost wax process, defects have occurred at various critical locations in the cast articles. These defects tend to occur at or adjacent to areas where there is a change in the configuration of the cast article. One particular type of defect which occurs in superalloy single crystal castings is a stripe-like condition known as zebra grains. These multiple, parallel, elongated grains tend to occur side by side on horizontal surfaces such as the platforms of single crystal blades or vanes. Of course, various other types of grain defects can occur in single crystal, columnar grained and equiaxed superalloy castings.
During the casting of a single crystal article from a superalloy, a crystal selector is utilized to select one of a plurality of crystals for growth into an article mold cavity. In U.S. Pat. No. 4,550,764 a body of insulation is provided to block the radiation of heat downwardly from a portion of a crystal selector passage to retard the spurious nucleation of crystals in the passage. The crystal selector disclosed in this patent has an upright primary growth passage and a horizontal secondary growth passage which cooperate to form a doubly-oriented single crystal casting. The use of crystal selectors having straight and helical passages is disclosed in U.S. Pat. No. 4,475,582. In U.S. Pat. No. 4,133,368 it is suggested that a helical crystal selector passage could be disposed in a preformed or precast ceramic insert. The insert has a thickness which provides a desired insulating effect in the area of the helical passage to improve thermal gradients and minimize spurious crystal nucleation.